Since the scientific discovery of light amplification by stimulated emission of radiation (a “laser”), lasers have found many practical, important, and wide-ranging applications. Lasers produce light at a very narrow spectral range at a singular wavelength and frequency. Laser light therefore differs significantly from ordinary visible light, known as white light, which contains light of a variety of different frequencies and wavelengths. Due to its narrow spectral frequency and fixed wavelength, laser light is typically referred to as monochromatic light. The singular frequency and wavelength characteristic of laser light make it ideal for many different practical applications where ordinary visible light is unsuitable.
The monochromatic light produced by lasers results from a phenomenon of quantum mechanics. A lasing medium is pumped with energy from an external source, and the electrons of the lasing medium are excited to a higher energy state. As the electrons return to the lower state, the energy which placed the electrons in the higher energy state is released through the emission of light. The characteristics of the light emitted are defined by the difference between the excited energy state and the lower energy state, causing light of a specific wavelength and frequency to be emitted. The frequency and wavelength of light are related to each other as a result of their product equaling the speed of light. Because the electron energy states of the lasing medium can not be generally altered, the frequency and wavelength of the laser beam are commonly fixed by the lasing medium.
Laser light having a frequency and wavelength other than the fundamental frequency emitted from the lasing medium might be more desirable for a particular application, but the cost of a laser with a specific type of lasing medium that emits that desired frequency and wavelength might be prohibitive, if such lasing medium was available to generate a fundamental frequency laser beam at the desired frequency.
The advent of harmonic generators have expanded the ability to achieve different wavelengths of laser light from a given lasing medium. A harmonic generator is formed from a crystal whose unit cell lacks a center of inversion. Such a crystal is also known as a nonlinear optical device. As light with a fundamental frequency laser beam passes through the harmonic generator crystal, light at a second harmonic frequency is formed. The second harmonic light is light which has a fundamental frequency two times higher than the fundamental frequency and which has a wavelength which is one half the wavelength of the fundamental frequency laser light. Thus for example, a second harmonic generator crystal can convert fundamental laser light with a wavelength of 1064 nm into light which possesses a wavelength of 532 nm (i.e., green light). Similarly, wavelengths of 355 and 266 nm can be generated from a 1064 nm laser source using third and fourth harmonic generator crystals, respectively.
Prior art techniques for implementing a second harmonic generator involve the use of folded resonator laser apparatus, as shown in FIGS. 1 and 2. A folded L-resonator laser apparatus 20 (FIG. 1) and a folded Z-resonator laser apparatus 22 (FIG. 2) use mirrors 24 and 26 (FIG. 1) and mirrors 28, 30 and 32 (FIG. 2), respectively, to pass a fundamental frequency laser beam 34 originating from a laser 36 through a SHG crystal 38. The fundamental laser beam 34 is passed through the SHG crystal 38 twice. The laser 36 generates the laser beam 34 of an initial fundamental wavelength and fundamental frequency that is reflected off the mirror 24 (FIG. 1) and mirrors 28 and 30 (FIG. 2). In each case, the mirror 24 (FIG. 1) and mirrors 28 and 30 (FIG. 2) are highly reflective of the fundamental laser beam 34, and therefore reflect substantially all of the incident light energy of the fundamental laser beam 34.
The reflected laser beam 34 passes through the SHG crystal 38. As the laser beam emerges from the SHG crystal 38, a certain portion of the fundamental beam 34 has been converted to a second harmonic laser beam 40a which possesses a frequency that has twice the fundamental frequency and half the wavelength of the fundamental beam 34. Not all of the fundamental laser beam 34 is converted into the second harmonic laser beam 40a, so the portion of the fundamental beam 34 that emerges from the SHG crystal 38 is indicated at 34a. The fundamental and second harmonic beams 34a and 40a are next reflected from the mirror 26 (FIG. 1) and mirror 32 (FIG. 2) back through the SHG crystal 38 again. The second pass of the fundamental beam 34a through the SHG crystal 38 again causes some of the energy of the fundamental beam 34a to be converted into the second harmonic beam 40b. The diminished energy of the fundamental beam which emerges from the SHG crystal 38 is represented by the fundamental beam 34b. The second harmonic beam 40b emitted from the SHG crystal 38 after the second pass of the fundamental beam through the SHG crystal has increased in power resulting from the pass of the fundamental beam 34a through the SHG crystal 38.
The fundamental beam 34b and the second harmonic beam 40b advance to the mirror 24 (FIG. 1) and the mirror 28 (FIG. 2). The mirror 24 (FIG. 1) and mirror 28 (FIG. 2) are dichroic mirrors, each with a dielectric coating (not shown) that selectively filters the fundamental frequency beams 34b from the second harmonic beam 40b. The dichroic mirrors 24 (FIG. 1) and 28 (FIG. 2) reflect the fundamental beam 34b back to the lasing medium of the laser 36, where its energy may be used to stimulate more light emission from the laser 36. However, the dichroic mirrors 24 (FIG. 1) and 28 (FIG. 2) pass the second harmonic beam 40b through the mirrors 24 (FIG. 1) and 28 (FIG. 2) without reflection. The second harmonic beam 40b is emitted from the laser apparatus 20 (FIG. 1) and 22 (FIG. 2) as a second harmonic laser beam 40 which can then be used for an intended purpose. The characteristics of the dielectric coating of the dichroic mirrors are selected to assure reflection and passage of the fundamental and second harmonic frequency laser beams.
By using the folded resonators 20 (FIG. 1) and 22 (FIG. 2), different frequencies of laser light beams can be obtained even though lasing medium of the laser 36 yields only a single laser beam 34 having a single fundamental frequency and a single fundamental wavelength. The amount of power of the second harmonic laser beam 40 is determined primarily by the efficiency of conversion of the SHG crystal 38. Limits exist as to the power of the second harmonic beam 40 that can be generated by the SHG crystal 38.
Increasing the intensity or power of the fundamental frequency laser beam 34 is one possible way to increase the power of the second harmonic beam 40, because more second harmonic energy will be converted from the higher energy fundamental frequency laser beam. Such an approach, however, has limited applicability because of physical limitations of the SHG crystal. The efficiency of conversion of the fundamental beam to the second harmonic beam is related to the square of the physical length of the SHG crystal through which the beam propagates. Increasing the length of the crystal increases the efficiency of conversion of the fundamental beam into the second harmonic beam. However, once the SHG crystal exceeds a specific but somewhat unpredictable length, further increasing the length of the SHG crystal will not lead to a further increase in the power of the second harmonic beam generated. Increasing the power of the fundamental beam above a certain power level will damage the SHG crystal through chemical decomposition of some of its constituents.
One technique of obtaining increased second harmonic power without damaging the SHG crystal is to use a two crystal resonator laser apparatus 42 shown in FIG. 3. The two crystal resonator laser apparatus 42 increases the power of the second harmonic beam by placing one SHG crystal 38a in series with a second SHG crystal 38b, and thereby causing the fundamental frequency laser beam to pass four times through the two SHG crystals 38a and 38b. Other than the use of the two series oriented SHG crystals 38, the two crystal resonator laser apparatus 42 is essentially similar to the folded resonator laser apparatus 20 (FIG. 1). The two SHG crystals 38a and 38b are aligned between the mirrors 24 and 26. Upon the fundamental laser beam 34 passing first through the SHG crystal 38a, the fundamental beam 34a emerges as a reduced power fundamental beam 34a and a second harmonic beam 40a is created. The beams 34a and 40a then pass through the second SHG crystal 38b, resulting in a reduced power fundamental beam 34b and an increased power second harmonic beam 40b. The beams 34b and 40b are reflected from the mirror 26 back through the second SHG crystal 38b. The emerging fundamental beam 34c has reduced power and the emerging second harmonic beam 40c has increased power. The beams 34c and 40c pass through the first SHG crystal 38a, and the fundamental beam 34d emerges with even less power while the second harmonic beam 40d emerges with even greater power.
The beams 34d and 40d impinge upon the dichroic mirror 24. The fundamental beam 34d is reflected back to the lasing medium of the laser 36, and the second harmonic beam 40d is emitted as the beam 40. Since the fundamental beam passes through the two SHG crystals 38a and 38b four times, more of the fundamental beam energy is converted into the second harmonic beam 40. While more second harmonic energy is obtained, using the two SHG crystals 38a and 38b in the laser apparatus 42 suffers from the difficulty of requiring a high level of optical precision to align and phase match the second harmonic beams 40a, 40b, 40c and 40d that are generated. Phase matching is required to obtain the increased second harmonic power, because any mismatch in phase results in a power reduction. The fundamental beam and the second harmonic beam become phase mismatched (and obtain different phase velocities) when they experience different indices of refraction as they propagate through the SHG crystals.
A technique known as critical phase matching takes advantage of the birefringent properties of SHG crystals to orient the SHG crystals with respect to the angle at which the fundamental beam impinges on the crystal. With the correct angle of impingement, the fundamental and second harmonic beams have the same indices of refraction and hence the same phase velocities, thereby achieving phase matching. Critical phase matching, however, suffers from the extreme precision required to successfully orient the crystal. For some SHG crystals, the efficiency at which the power of the fundamental beam is converted to the second harmonic beam can be reduced by as much as 50% if the crystal is misaligned by only 1-3 mrad (0.06-0.17°). Accordingly, if the two crystals are slightly misaligned, the benefits of the multiple pass are substantially diminished. Furthermore, any misalignment of one SHG crystal compounds the misalignment of the second SHG crystal.
Critical phase matching may also be achieved by heating the SHG crystals to elevated temperatures. While heating to achieve phase matching may be acceptable under some circumstances, such as in the laboratory or in some highly controllable industrial or manufacturing processes, heating may be inappropriate in other situations, such as in consumer products.